Dear fellows,
I'm working with vein dol-qz FI, mainly hosted by Fe-dolomites. Despite
their low average salinity (near zero), in some of them I found eutectic
temperatures as low as -52 C, compatible with the system CaCl2-H2O.

However, I have a negligible contribution of Ca for the hydrothermal chemistry.
I want to test thus a MgCl-H2O thermodynamic model to fit the data, and
didn't found a reference about it. Do you have any suggestion?

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MgCl2-H2O only goes to eutectic at -35 degrees -- see Chapt. 8 in my 1984
book Fluid Inclusions.

Edwin Roedder

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Fellow inclusionists:

Over the last several years we have been doing much work on various water-salt
systems using synthetic fluid inclusions. We have looked at many systems
including H2O-CaCl2-NaCl, H2O-NaCl-MgCl2, H2O-FeCl2, H2O-FeCl2-NaCl, etc.
In all of these systems and others, we see "something happen"
at temperatures well below the expected eutectic temperatures. These changes
in appearance of the inclusion are reproducible, and occur in the same temperature
range for each composition. Yet, I am fairly certain that we are NOT seeing
eutectic melting. Rather, I believe that it is either a metastable (but
reproducible) eutectic or, more likely, some type of solid state transition
or glass transition. Any of you who have worked with synthetic H2O-CO2 fluid
inclusions have probably seen the very obvious and reproducible change in
appearance of the inclusions that occurs at about -90 to -80 during heating
from very low temperatures. This change in appearance is apparently a solid
state transformation and is not a melting event.

All of these observations that we have made using synthetic fluid inclusions
make me very skeptical of low eutectic temperatures that are being reported
more and more commonly in the fluid inclusion literature. In the past, with
poorer quality optics and cooling stages that either could not be cooled
to very low temperatures or in which the temperature could not be controlled
very easily, these "phase changes" would have been easily overlooked.
With today's better equipment, we are seeing more subtle changes in the
fluid inclusions, but I am not completely convinced that most of these changes
are in fact eutectic melting events.

I would be intersted to hear what other people think about this, especially
those of you who have extensive experience using synthetic fluid inclusions
in which the real eutectic is known.

Bob Bodnar

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Fellow inclusionists:

I completely agree with Bob Bodnar's comments about observations of low
temperature phase behavior in aqueous inclusions. In a previous discussion
on this list I said that I'm convinced that many "eutectic" melting
observations are really devitrification of saline, aqueous glass. The glass
transition temperature in saline aqueous systems is well documented in the
chemistry literature. Austin Angell has published a number of papers on
this subject (there is a citation for Angell and Kanno, in the Goldstein
and Reynolds book. I don't remember it off hand.)

If anyone wants to understand the low temperature phase transitions that
can be observed in aqueous inclusions, and "practice" making the
observations, I recommend getting some halite, finding some large inclusions,
and carefully observing the transitions. After you've seen the stuff in
the big inclusions, then try making the observations on the small inclusions
in the same sample. This can be a humbling experience. The interpretation
is not easy. Keep in mind that there is lots of metastability as Ed Roedder
pointed out many years ago.
In any case, have fun with microthermometry.

Bob Burruss

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A possibly informative alternative to Bob Burruss's suggestion (though not
as simple in sample prep.). aqueous solutions of Ca(NO3)2 are good 'glass'
formers (and thus some of the Angell work Bob cited deals with Ca(NO3)2).
It has a much higher eutectic than CaCl2 so IF it is possible to realize
any of its metastable eutectics vs glass transitions/devitrification, the
eutectics should be much more easily observable/attainable. Its solubility
also increases very rapidly with temperature (increasing from about 8 molal
at 25 C to >24 molal at 100 C). There are several hydrate phases which
melt congruently (the tetrahydrate at about 55 C) and incongruently and
they have the nasty tendency to metastably persist. So while the system
isn't particularly relevant geologically, it could be a decent teaching
tool in that it's phase behavior is messy enough to make things interesting
(or frustrating) while retaining chemical simplicity.

For you folks that do Raman and IR work there has been a fair amount of
work on Ca(NO3)2(aq) though I can't think of any at <0 C. Seems to me
that while fine grained crystalline aggregates and glasses in flincs can
be visually indistinguishable they would have vastly different Raman characteristics
(at least for this case). One of you folks can tell me whether a glass and
a melt (e.g. the tetrahydrate) would be distinguishable or for that matter
are physically distinct.

Charlie Oakes

P.S. I believe that I sent an earlier reply, inadvertantly, ONLY to Rogerio
(I know some of you hate it when I do that). Anyway, I gave him some references
to thermo models for MgCl2 and a recent paper by Dubois and Marignac which
deals with MgCl2 in flincs. If anyone wants the refs - ask. Or if he wants
to forward the list back to the list 'tis fine with me.

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Dear all,

I just want to add some comments to the discussion about the meaning of
eutectic temperature. I completely agree with Bob Burruss concerning the
behaviour of water-salt(s) systems, when Bob is writing that many (supposed)
eutectic behaviour are due to devitrification. I have to mention the work
of Vuillard and Kessis (1960) about the (de-)vitrification in the H2O-LiCl
system :

"Vitrification is observed in binary mixtures in the eutectic composition
range in systems where the eutectic temperature is low. The solid phase
is supposed to appear in a very viscous medium [...]. The vitrification
temperature is more or less constant for a given system and, when always
operating in the same way, it is possible to obtain for this temperature
a reproducibilitty comparable to the melting temperature."

During heating, vitrification is followed by a devitification ("recuit")
which consists of the crystallization of the expected stable phases. However
the authors are talking about one to two days to reach the complete crystallization!

I think the best way to measure correct (or not too false) eutectic temperature
is to use the sequential freezing (Haynes, 1985), i.e. when one big ice
crystal is present. It is surprising to note that the (supposed) eutectic
behaviour occurs differently (i.e. at different temperatures) when 'eutectic
temperature' is measured during the initial freezing/heating run or during
the sequential freezing procedure, and sometimes during successive sequential
freezing runs ! This supposes that in the case of systems with succesive
hydrates, metastable eutectics are possible. This fact was never observed
in simple binary systems such as H2O-KCl, H2O-CsCl or H2O-RbCl (Dubois et
al., 1993, 1997). An application of Haynes' technique to the determination
of precise eutectic temperatures is given in the former citation.

Concerning the reference given by Charlie Oakes about the H2O-NaCl-MgCl2
system (which was the initial question !), the complete reference of my
communication is:

in reponse to the two Bob's, I think a practical solution to the problem
of apparent low eutectics, and an alternative to the sequential freezing
of Haynes mentioned by Michel Dubois, is to attempt to identify the burst
of rapid melting which commonly occurs at the stable eutectic (e.g. see
description by Davis et al., Geochimica, 54 p. 600.). In many natural inclusions
where good bulk analyses are available, a marked increase in melting behaviour
(crystal coarsening, changes in appearance of vapour bubble, appearance
of patches of vapour etc.) is observed at the stable eutectic expected for
the dominant salts present in solution.

With regard to Bob Bodnar's enquiry about synthetic inclusions we have observed
clear melting below the stable eutectic in synthetic inclusions in the KCl-H2O
system (-10.7 C reported by Hall et al., 1988, Econ. Geol., 83, 197-202).
In fact, euhedral solid (presumed to be sylvite) was observed to coexist
with ice and liquid for several hours without noticeable change at temperatures
between -14.3 and -11.3 C (I have some good photographs of this). It is
possible this indicates the presence of a KCl hydrate, only reported in
the Russian literature.

Dr Jamie Wilkinson

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Fellow inclusionists:

I enjoyed recent discussions on the question raised by Rogerio Kiwitko.
I would like to point out that the best reference (and discussion) for the
MgCl2-H2O system is the abstract by Ron Spencer and Tim Lowenstein on "Phase
Equlibria in the System MgCl2-H2O" published in PACROFI IV Program
and Abstracts (Lake Arrowhead, California, May 21-25, 1992) p. 138-141.
In that abstract, many eutectic (stable and metastable) temperatures were
listed, including ice-MG12 (-33.6 C), MG12-MG8 (-16.8C), MG12-BISCH (-19.4C),
ice-BISCH (near -80 C), ice-MG8 (near -50C).

Reaction between bischofite and ice beginning at metastable eutectic (no
liquid was observed) was also suggested as possible explanations for the
recrystalization of solids within frozen fluid inclusions on warming between
-80 and -70 C. For the ternary system NaCl-MgCl2-H2O, see Davis et al. (1990,
GCA 54, 591-601).

I-Ming Chou

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Jamie and others:

You are right about the burst of melting if the inclusion has a fairly high
salinity. But, remember that the amount of liquid generated at the eutectic
is directly related to the salinity (i.e., the lever rule). If you have
a composition close to the eutectic salinity you get an obvious burst of
melting, but if the salinity is very low the amount of melting at the eutectic
may be so small that a melting event may not be obvious, especially in small
inclusions.

Bob Bodnar

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Bob,

your comment is appreciated, nonetheless the same also applies to metastable
hydrate melting phenomena - I think the relative increase in melting rate
which occurs at the stable eutectic can be observed even if metastable melting
or glass transition effects have taken or are taking place (at least for
inclusions of reasonable size and/or salinity, typically in excess of say
10wt%). In small and/or very dilute inclusions I don't think that it is
possible to confidently observe either stable or metastable eutectic melting
for the reason you give. There's no solution for these inclusions...

Jamie Wilkinson

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I agree completely.

Bob Bodnar

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Dear fellows,

Thank you for the precious suggestions about the eutectic misleads

I have faced another doubt in my studies, regarding ThCO2 measuring. In
continuous freezing/heating, some inclusions showed CO2 homogenization in
the presence of clathrate. Stepped cooling/heating measures of the same
inclusions provide ThCO2 without clathrate 9 C lower, as an average.

What is the meaning of a ThCO2 measured with clathrate, and its consistently
higher T values?